Variable capacity piston type automotive refrigerant compressors typically have one sided pistons driven by a nutating wobble plate, sometimes called a socket plate. Because of the fact that the pistons are one sided, the potential exists for changing the stroke of the pistons, and thus the capacity of the compressor, by changing the angle that the plate makes with the central co axis of the drive shaft and compressor housing. The structure that allows the angle to change is an interconnection between the hub and plate that includes a kidney shaped slot and pin that can slide through the slot. A variable force balance on the pistons is created by a changing differential between the crankcase pressure, which presses on the back of the pistons, and the suction pressure, which acts on the front of the pistons, controlled through an interconnecting valve. As the pressure balance acts on the pistons, the pistons transfer the changing net force to the plate to change its angle relative to the central axis, and the pin slides passively through the slot to a new rest position. The driving connection between hub and plate remains one to one, however, regardless of the movement of the pin through the slot.
Two different basic structural relationships between the nutating plate and the central drive shaft are found in the prior art. In one basic design, the nutating plate that drives the pistons itself rotates about a spherical bearing on the shaft as it nutates, rotating one to one with the hub. An example may be seen in U.S. Pat. No. 4,664,604. In another basic design, the nutating socket plate does not rotate about the shaft axis one to one with the hub and shaft. Instead, the driving connection between the hub and socket plate is made indirectly, through an intermediate journal interposed between the socket plate and hub. The journal turns one to one with the hub and shaft, and imparts the same nutating motion to the socket plate, but rotates freely relative to the stationary socket plate on thrust and radial bearings. The pin and slot assembly that allows angular change interconnects the journal and hub, rather than directly interconnecting the socket plate and hub. The journal, in turn, is allowed to shift axially over the shaft as its angle changes relative to the shaft by virtue of being pivotally mounted to a sleeve that slides axially on the drive shaft. When the journal changes angle, it transmits the same angle change to the socket plate, which remains parallel to the journal. While the journal turns one to one with the shaft driven hub, the socket plate is restrained from rotating by a small spherical bearing installed near its outer edge, which slides back and forth along a guide pin that is parallel to the housing central axis. A typical example of the guide pin type of socket plate anti rotation mechanism is shown in co-assigned U.S. Pat. No. 4,428,718.
A drawback of the basic guide pin torque restraint design is the fact that the socket plate is subjected to a twice per rotation torsional oscillation as it rotates, due to the angular mismatch between the socket plate and shaft-hub axes. This in turn causes compressor vibration. Another co-assigned U.S. Pat. No. 5,129,752 recognized the problem, and proposed a novel replacement for the guide pin type of socket plate torque restraint assembly. It was replaced with a ball and track type of constant velocity joint, known as an Rzeppa joint, which is normally used for the very different purpose of transmitting torque from a rotating driving shaft to a driven shaft. Here, however, by fixing the outer joint race coaxially to the socket plate, and by securing the inner joint race so as to remain coaxial to the hub, but non rotatable relative to the compressor housing, the socket plate is restrained against rotation in a way that balances out the torsional oscillation. Critical to the design is the structure that allows the inner joint race to remain coaxial to the hub without rotating. This is a totally separate shaft, called an anti rotational shaft 52, which is coaxial to the drive shaft. The anti rotational shaft 52 also must be axially slidable on splines within the compressor housing, and spring loaded toward a return position. This allows the shaft and the inner joint race to axially shift when the angle of the socket plate changes, providing the same function that the sliding sleeve in a conventional socket plate design does. This extra shaft, as well as the Rzeppa joint, adds a good deal of structural complexity, however, compared to a simple guide pin and spherical bearing.